U.S. patent application number 17/062636 was filed with the patent office on 2021-04-15 for methods of manufacturing thin, high density nonwoven separators for energy storage devices.
The applicant listed for this patent is Testa Mattia, Brian G. Morin, Giovanni Schnelle. Invention is credited to Testa Mattia, Brian G. Morin, Giovanni Schnelle.
Application Number | 20210111464 17/062636 |
Document ID | / |
Family ID | 1000005298594 |
Filed Date | 2021-04-15 |
United States Patent
Application |
20210111464 |
Kind Code |
A1 |
Morin; Brian G. ; et
al. |
April 15, 2021 |
METHODS OF MANUFACTURING THIN, HIGH DENSITY NONWOVEN SEPARATORS FOR
ENERGY STORAGE DEVICES
Abstract
An insulating (nonconductive) microporous nonwoven polymeric
battery separator comprised of a single layer of enmeshed
microfibers and nanofibers and supercalendered to extremely thin
dimensions and high densities is provided. Such a separator accords
the ability to not only attune the porosity and pore size to any
desired level through a single nonwoven fabric, but provide further
benefits in terms of further reduced pore size, reduced electrolyte
level requirements, and reduced total volume of the subject battery
cell itself. As a result, the inventive separator permits a high
strength material with low porosity and low pore size to levels
previously unattained. The separator, a battery including such a
separator, the method of manufacturing such a separator, and the
method of utilizing such a separator within a battery device, are
all encompassed within this invention.
Inventors: |
Morin; Brian G.;
(Greenville, SC) ; Mattia; Testa; (Scaer, FR)
; Schnelle; Giovanni; (Nidda, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Morin; Brian G.
Mattia; Testa
Schnelle; Giovanni |
Greenville
Scaer
Nidda |
SC |
US
FR
DE |
|
|
Family ID: |
1000005298594 |
Appl. No.: |
17/062636 |
Filed: |
October 5, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15054120 |
Feb 25, 2016 |
10797286 |
|
|
17062636 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 50/411 20210101;
H01M 10/0525 20130101; H01M 50/44 20210101 |
International
Class: |
H01M 50/44 20060101
H01M050/44; H01M 10/0525 20060101 H01M010/0525; H01M 50/411
20060101 H01M050/411 |
Claims
1. A method of manufacturing a single-layer polymeric battery
separator, said method comprising the steps of: providing a
plurality of polymeric microfibers having a maximum length of 25 mm
and a minimum size of 2 microns; providing a plurality of polymeric
nanofibers having a maximum length of 25 mm and a maximum size of
700 nanometers; subjecting said plurality of microfibers and
plurality of nanofibers simultaneously to a wetlaid nonwoven
fabricating method such that said polymeric microfibers enmesh in a
non-uniform pattern with interstices between said microfibers and
said polymeric nanofibers become entangled with said microfibers
and with said other nanofibers such that said nanofibers are
introduced within said interstices between said microfibers as well
as on the surface of the substrate formed from said plurality of
polymeric microfibers; and subjecting said enmeshed structure to a
supercalendering procedure, wherein said procedure entails contact
with at least three separate calendering nips, and wherein each
calendering nip applies a pressure of at least 500 lbs/inch.
2. The method of claim 1 wherein said supercalendered enmeshed
structure exhibits a maximum thickness of 25 microns and a maximum
porosity of 45%.
3. The method of claim 2 wherein said supercalendered enmeshed
structure exhibits a maximum mean flow pore size of 0.7
microns.
4. The method of claim 1 wherein said supercalendered enmeshed
structure exhibits a maximum thickness of 20 microns.
5. The method of claim 1 wherein said supercalendered enmeshed
structure exhibits a maximum thickness of 15 microns.
6. The method of claim 1 wherein said supercalendered enmeshed
structure exhibits a maximum thickness of 12 microns.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of pending U.S. patent
application Ser. No. 15/054,120, filed on Feb. 25, 2016, the
entirety of which is herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to an insulating
(nonconductive) microporous nonwoven polymeric battery separator
comprised of a single layer of enmeshed microfibers and nanofibers
and supercalendered to extremely thin dimensions and high
densities. Such a separator accords the ability to not only attune
the porosity and pore size to any desired level through a single
nonwoven fabric, but provide further benefits in terms of further
reduced pore size, reduced electrolyte level requirements, and
reduced total volume of the subject battery cell itself. As a
result, the inventive separator permits a high strength material
with low porosity and low pore size to levels previously
unattained. The separator, a battery including such a separator,
the method of manufacturing such a separator, and the method of
utilizing such a separator within a battery device, are all
encompassed within this invention.
BACKGROUND OF THE INVENTION
[0003] Batteries have been utilized for many years as electrical
power generators in remote locations. Through the controlled
movement of electrolytes (ions) between electrodes (anode and
cathode), a power circuit is generated, thereby providing a source
of electricity that can be utilized until the electrolyte source is
depleted and no further electrical generation is possible. In more
recent years, rechargeable batteries have been created to allow for
longer lifetimes for such remote power sources, albeit through the
need for connecting such batteries to other electrical sources for
a certain period of time. All in all, however, the capability of
reusing such a battery has led to greater potentials for use,
particularly through cell phone and laptop computer usage and, even
more so, to the possibility of automobiles that solely require
electricity to function.
[0004] Such batteries typically include at least five distinct
components. A case (or container) houses everything in a secure and
reliable manner to prevent leakage to the outside as well as
environmental exposure inside. Within the case are an anode and a
cathode, separated effectively by a separator, as well as an
electrolyte solution (low viscosity liquid) that transport over
and/or through the separator between the anode and cathode. The
rechargeable batteries of today and, presumably tomorrow, will run
the gamut of rather small and portable devices, but with a great
deal of electrical generation potential in order to remain
effective for long periods between charging episodes, to very large
types present within automobiles, as an example, that include large
electrodes (at least in surface area) that must not contact one
another and large amounts of electrolytes that must consistently
and constantly pass through a membrane to complete the necessary
circuit, all at a level of power generation conducive to providing
sufficient electricity to run an automobile engine. As such, the
capability and versatility of battery separators in the future must
meet certain requirements that have yet to be provided within the
current industry.
[0005] Generally speaking, battery separators have been utilized
since the advent of closed-cell batteries to provide necessary
protection from unwanted contact between electrodes as well as to
permit effective transport of electrolytes within power generating
cells. Typically, such materials have been of film structure,
sufficiently thin to reduce the weight and volume of a battery
device while imparting the necessary properties noted above at the
same time. Such separators must exhibit other characteristics, as
well, to allow for proper battery function. These include chemical
stability, suitable porosity of ionic species, effective pore size
for electrolyte transfer, proper permeability, effective mechanical
strength, and the capability of retaining dimensional and
functional stability when exposed to high temperatures (as well as
the potential for shutdown if the temperature rises to an
abnormally high level).
[0006] In greater detail, then, the separator material must be of
sufficient strength and constitution to withstand a number of
different scenarios. Initially, the separator must not suffer tears
or punctures during the stresses of battery assembly. In this
manner, the overall mechanical strength of the separator is
extremely important, particularly as high tensile strength material
in both the machine and cross (i.e., transverse) directions allows
the manufacturer to handle such a separator more easily and without
stringent guidelines lest the separator suffer structural failure
or loss during such a critical procedure. Additionally, from a
chemical perspective, the separator must withstand the oxidative
and reductive environment within the battery itself, particularly
when fully charged. Any failure during use, specifically in terms
of structural integrity permitting abnormally high amounts of
electrolyte to pass or for the electrodes to touch, would destroy
the power generation capability and render the battery totally
ineffective. Thus, even above the ability to weather chemical
exposure, such a separator must also not lose dimensional stability
(i.e., warp or melt) or mechanical strength during storage,
manufacture, and use, either, for the same reasons noted above.
[0007] Simultaneously, however, the separator must be of proper
thickness to, in essence, facilitate the high energy and power
densities of the battery, itself. A uniform thickness is quite
important, too, in order to allow for a long life cycle as any
uneven wear on the separator will be the weak link in terms of
proper electrolyte passage, as well as electrode contact
prevention. The ability, however, to provide an extremely thin,
uniform dimension, within such battery separators has proven to be
rather difficult, particularly since a thickness reduction of an
already thin structure tends to compromise separator strength. Film
separator structures may accord a certain thin dimension due to
facilitation of production of such structures; nonwoven separators,
to the contrary, are difficult to manufacture at a thin dimension
without losing integrity and, as is generally accepted, increasing
the pore sizes therein.
[0008] Additionally, with regard to pore sizes, such a battery
separator must exhibit proper porosity and pore sizes to accord,
again, the proper transport of ions through such a membrane (as
well as proper capacity to retain a certain amount of liquid
electrolyte to facilitate such ion transfer during use). The pores
themselves should be sufficiently small to prevent electrode
components from entering and/or passing through the membrane, while
also allowing, again, as noted above, for the proper rate of
transfer of electrolyte ions therethrough. As well, uniformity in
pore sizes, as well as pore size distribution, provides a more
uniform result in power generation over time as well as more
reliable long-term stability for the overall battery as, as
discussed previously, uniform wear on the battery separator, at
least as best controlled in such a system, allows for longer
life-cycles. It additionally can be advantageous to ensure the
pores therein may properly close upon exposure to abnormally high
temperatures to prevent excessive and undesirable ion transfer upon
such a battery failure (i.e., to prevent fires and other like
hazards). Thus, providing uniformly small pore sizes (and thus
proper porosity measurements for such a purpose) within a thin,
dense nonwoven structure has yet to be explored. Film structures,
again, may be manufactured to certain dimensions, but porosity
reductions are designed in for such separators, rather than
produced or at least modified through further treatments past
initial manufacture. In any event, in terms of nonwoven separators,
there remains a drive for very low pore sizes, at least to provide
beneficial protections in terms of electrode contact.
[0009] As well, the pore sizes and distributions may increase or
decrease the air resistance of the separator, thus allowing for
simple measurements of the separator that indicate the ability of
the separator to allow adequate passage of the electrolyte present
within the battery itself. For instance, mean flow pore size can be
measured according to ASTM E-1294, and this measurement can be used
to help determine the barrier properties of the separator. Thus,
with low pore size, the rigidity of the pores themselves (i.e., the
ability of the pores to remain a certain size during use over time
and upon exposure to a set pressure) allows for effective control
of electrode separation as well, as noted above. More importantly,
perhaps, is the capability of such pore size levels to limit
electrolyte permeability in order to reduce the chances of crystal
formation on an anode (such a lithium crystals on a graphite anode)
that would impair the generation of the necessary circuit and
deleteriously impact the power generation capability of the battery
over time. The smaller the pore sizes within the dimensional stable
thin and dense separator would ostensibly provide such benefits and
reduce, or at least retard, dendritic formations on the electrodes
(which could cause shorts within the circuit).
[0010] Furthermore, the separator must not impair the ability of
the electrolyte to completely fill the entire cell during
manufacture, storage and use. Thus, the separator must exhibit
proper wicking and/or wettability during such phases in order to
ensure the electrolyte in fact may properly generate and transfer
ions through the membrane; if the separator were not conducive to
such a situation, then the electrolyte would not properly reside on
and in the separator pores and the necessary ion transmission would
not readily occur, at least in theory. The smaller the separator,
the better, in other words. Providing a strong, thin, and dense
structure would be highly desirable, certainly, for this
purpose.
[0011] The general aim of an effective battery separator, then, is
to provide low air resistance and, simultaneously, very low pore
size, in order to accord a material that drastically reduces any
potential for electrode contact, but with the capability of
controlled electrolyte transport from one portion of the battery
cell to the other (i.e., closing the circuit to generate the needed
electrical power). Currently, such properties are not effectively
provided in tandem. For instance, Celgard has disclosed and
marketed an expanded film battery separator with very low pore
size, which is very good, as noted above; however, the
corresponding air resistance for such a material is extremely high,
thus limiting the overall effectiveness of such a separator. Even
with a thin structure, then, there are deleterious results that may
render such separators less effective and reduce certain durability
measurements. To the contrary, duPont commercializes a nanofiber
nonwoven membrane separator that provides very low air resistance,
but with very large pore sizes therein. Thus, dendritic formations
of the electrolyte on the anode, at least, may cause problems, as
well. Additionally, the overall mechanical strengths exhibiting by
these two materials are very limiting; the Celgard separator has
excellent strength in the machine direction, but nearly zero in the
cross (transverse) direction. Such low cross direction strength
requires very delicate handling during manufacture, at least, as
alluded to above. The duPont materials fare a little better, except
that the strengths are rather low in both directions, albeit with a
cross direction that is higher than the Celgard material. In
actuality, the duPont product is closer to an isotropic material
(nearly the same strengths in both machine and cross directions),
thus providing a more reliable material in terms of handling than
the Celgard type. However, the measured tensile strengths of the
duPont separator are quite low in effect, thus relegating the user
to carefully maneuvering and placing such materials during
manufacture as well. Likewise, the dimensional stability of such
prior battery separators are highly suspect due to these tensile
strength issues, potentially leading to materials that undesirably
lose their structural integrity over time when present within a
rechargeable battery cell.
[0012] New types of battery separators have been provided the
industry in terms of single layer nonwovens having enmeshed
microfiber and nanofiber constituents. Such structures allow,
depending on certain manufacturing steps and procedures, a user to
dial in a desired level of porosity with effective isotropic
strength levels. Such separators are effective in terms of air
resistance, as well, providing highly desirable structures within
the lithium ion and other like battery markets. A drawback does
exist, however, in terms of thickness and possible lower pore size
levels. As single layer structures these bi-component fiber
nonwovens are quite thin and permit a certain increase in battery
cell component volume as a result. However, the thicknesses of such
structures may require a certain level of material introduction
that may compromise certain battery effectiveness overall.
[0013] Thus, there still exists a need to provide a battery
separator that provides simultaneously low air resistance and low
pore size, as well as high tensile strength overall and at
relatively isotropic levels, all while exhibiting proper chemical
stability, structural integrity, dimensional stability, and ease in
manufacture, and at a thickness level that accords maximum volume
within a battery cell. Additionally, a manner of producing battery
separators that allows for achieving targeted property levels (such
as a specific range of pore sizes and/or a specific range of air
resistance measurements) through minor modifications in
manufacturing would permit greater versatility to meet battery
manufacturer requirements on demand; currently, such a
manufacturing method to such an extent has yet to be explored
throughout the battery separator industry. As such, an effective
and rather simple and straightforward battery separator
manufacturing method in terms of providing any number of membranes
exhibiting such versatile end results (i.e., targeted porosity and
air resistance levels through processing modifications on demand)
as well as necessary levels of mechanical properties, heat
resistance, permeability, dimensional stability, shutdown
properties, and meltdown properties, is prized within the
rechargeable battery separator industry; to date, such a material
has been unavailable.
Advantages and Summary of the Invention
[0014] A distinct advantage of the present invention is the ease in
manufacturing through a wetlaid nonwoven fabrication process
followed by a supercalendering procedure. Another distinct
advantage is the resulting capability of providing any targeted
level of pore size, porosity, and air resistance, through the mere
change in proportions of component fibers utilized during the
fabrication process, as well as through the compression forces
applied through such a supercalendering process. Yet another
advantage of this inventive battery separator is the increased
energy density provided through such a supercalendered structure,
with simultaneous reduction of pore sizes from its initial
manufactured state, unexpectedly. The ability of such a
supercalendered structure to reduce dendrite formation on
electrodes in relation to electrolyte is yet another advantage
herein. The ability to reduce the total volume of a battery cell
with a thin, dimensionally stable separator of this type, is
another significant advantage, as well. The inventive separator to
provide contemporaneous low air resistance and extremely low pore
sizes with a stronger, yet thinner, structure in comparison with an
initially manufactured nonwoven structure, is still a further
advantage of this invention. Yet another advantage of this
inventive battery separator is the provision of a specifically
non-conductive (and thus insulating) fabric that does not allow
transmission of electrical charge through the separator body, but
solely through the transport of charged ions through the pores
present within its structure. Yet another advantage is the high
porosity of the material, allowing the user to reduce the amount of
electrolyte actually needed for proper battery function.
[0015] Accordingly, this invention pertains to a polymeric battery
separator comprising a nonwoven combination of microfibers and
nanofibers, wherein said separator provides sufficient porosity for
electrolyte ion transfer therethrough and suitable prevention of
electrode contact through a single layer of said nonwoven
combination, wherein said separator exhibits a maximum thickness of
25 microns (preferably 20, more preferably 15, and most preferably
12 microns), a maximum porosity of 45% (preferably 40, more
preferably 35, and most preferably 30%), a maximum mean flow pore
size of 0.7 microns (preferably 0.6, more preferably 0.5, and most
preferably 0.4 microns), a minimum tensile strength of 2
kN/cm.sup.2 (preferably 2.5, more preferably 3.0, and most
preferably 3.5 kN/cm.sup.2), a minimum tensile strength of 0.6 kN/m
(preferably 0.7, more preferably 0.8, and most preferably 0.85
kN/m), and a maximum apparent density of 0.7 g/cm.sup.3 (preferably
0.8, more preferably 0.8, and most preferably 0.85 g/cm.sup.3).
Also encompassed herein is a method of manufacturing a single-layer
polymeric battery separator, said method comprising the steps
of:
[0016] providing a plurality of polymeric microfibers having a
maximum length of 25 mm and a minimum size of 2 microns;
[0017] providing a plurality of polymeric nanofibers having a
maximum length of 25 mm and a maximum size of 700 nanometers;
[0018] subjecting said plurality of microfibers and plurality of
nanofibers simultaneously to a wetlaid nonwoven fabricating method
such that said polymeric microfibers enmesh in a non-uniform
pattern with interstices between said microfibers and said
polymeric nanofibers become entangled with said microfibers and
with said other nanofibers such that said nanofibers are introduced
within said interstices between said microfibers as well as on the
surface of the substrate formed from said plurality of polymeric
microfibers; and
[0019] subjecting said enmeshed structure to a supercalendering
procedure, wherein said procedure entails contact with at least
three separate calendering nips, and wherein each calendering nip
applies a pressure of at least 500 lbs/inch. A battery including
such an insulating separator as above and/or manufactured through
the defined process is likewise encompassed within this invention,
as is the method of utilizing such a battery to generate
electricity in a rechargeable device.
[0020] Throughout this disclosure, the term microfiber is intended
to mean any polymeric fiber exhibiting a width that is measured in
micrometers, generally having a fiber diameter greater than 1000
nm, but also greater than 3000 nm, or even greater than 5000 nm or
possibly even greater than 10,000 nm, up to about 40 microns. As
well, the term nanofiber is intended to mean any polymeric fiber
exhibiting a width that is measured in nanometers, generally having
a diameter less than 1000 nm, but possibly less than 700 nm, or
even less than 500 nm or possibly even less than 300 nm. As well,
the term insulating in intended to indicate no appreciable degree
of electrical conductivity, and thus the inventive fabric structure
does not permit electrical charge throughout the fabric body, but
only through the passage of electrolytic ions through the pores
present therein.
[0021] Such a combination of microfibers and nanofibers has yet to
be investigated within the battery separator art, particularly in
terms of the capability of providing a single-layer nonwoven fabric
of the two base components for such a purpose. The microfiber
constituent may be of any suitable polymer that provides the
necessary chemical and heat resistance alluded to above, as well as
the capability of forming a microfiber structure. As well, such a
microfiber may also be fibrillated (or treated in any other like
manner, such as through plasma exposure, and the like) during or
subsequent to fiber formation in order to increase the surface area
thereof to facilitate the desired entangling between a plurality of
such microfibers during a nonwoven fabrication process. Such
polymeric components may thus include acrylics such as
polyacrylonitrile, polyolefins such as polypropylene, polyethylene,
polybutylene and others including copolymers, polyamides, polyvinyl
alcohol, polyethylene terephthalate, polybutylene terephthalate,
polysulfone, polyvinyl fluoride, polyvinylidene fluoride,
polyvinylidene fluoride-hexafluoropropylene, polymethyl pentene,
polyphenylene sulfide, polyacetyl, polyurethane, aromatic
polyamide, semi-aromatic polyamide, polypropylene terephthalate,
polymethyl methacrylate, polystyrene, cellulosic polymers (rayon,
as one non-limiting example), polyaramids, including para-aramids
and meta-aramids, and blends, mixtures and copolymers including
these polymers. Polyacrylates, cellulosic polymers, and polyaramids
are potentially preferred.
[0022] The fibers may also be pre-treated with adhesives to
effectuate the desired degree of contact and dimensional stability
of the overall nonwoven structure subsequent to fabrication.
[0023] Additionally, the microfibers may be selected in terms of
individual fiber properties to provide combinations of materials
that accord desirable characteristics to the overall battery
separator. Thus, since poly-aramid, meta-aramid, and cellulosic
fibers provide excellent heat resistance and certain strength
benefits, such fibers may be incorporated individually (as wet-laid
constituents, for example) or in combination through entanglement
or other means. Such fibers must be of sufficient length to impart
the necessary strength to the overall separator but short enough to
permit proper incorporation (such as, again, for instance, within a
wet-laid procedure). For instance, they may preferably be longer
than 0.5 mm, more preferably longer than 1 mm, and most preferably
longer than 2 mm.
[0024] Microfibers or nanofibers may preferentially be of a
material that will melt or flow under pressure or high temperature.
It is of particular benefit to have one constituent which will melt
or flow at a temperature that is lower than the other constituents.
For example, polyester microfibers can be made to flow at
temperatures approaching the melt temperature of 260.degree. C.
Additionally, polyacrylonitrile microfibers or nanofibers can be
made to flow under high pressure and temperature. Cellulose, rayon,
aramid, and other micro- or nanofibers will not flow under these
temperatures. Thus, a combination of materials comprising at least
one fiber that will flow under high temperature and/or pressure and
at least one fiber that will not flow under the same temperature
and/or pressure will enable the first fiber to bond the other
fibers together, imparting additional strength to the nonwoven
separator.
[0025] The nanofibers may thus be of any like polymer constituency
in order to withstand the same types of chemical and high
temperature exposures as for the microfibers. Due to their size,
there is no requirement of post-manufacture treatment of such
nanofiber materials to accord any increase in entanglement on the
produced nonwoven surface or within the interstices thereof.
Importantly, however, is the necessity that the nanofibers combine
with the microfibers under a sufficiently high shear environment to
accord the desired introduction of such nanofibers onto and within
the resultant microfiber nonwoven substrate simultaneously with
actual nonwoven fabrication itself. In other words, upon the
provision of both types of fiber materials within the nonwoven
production process, the manufacturer should accord a sufficient
amount of mixing and under high shear conditions to best ensure the
proper degree of entanglement between the different fiber types to
form the desired single-layer fabric structure. As well, the
fabrication method is potentially preferred as a wetlaid nonwoven
procedure in addition to the high shear type, ostensibly to best
ensure the proper introduction and residual location of nanofibers
within the microfiber interstices. With an increased water flow
during manufacture, the extremely small nanofibers will be drawn
into such interstices at a greater rate than with a dry
entanglement method, thereby according the aforementioned
interstice fill capability. The resultant nonwoven structure would
thus exhibit greater uniformity in terms of thickness, porosity,
and, most importantly, pore sizes, therein.
[0026] Other methods of nonwoven sheet manufacture which enable the
entanglement of a combination of nanofibers and microfibers may
also be used to create the inventive battery separators. Such
methods include carding, cross lapping, hydroentangling, air laid,
needlepunch, or other methods that enable the microfibers to form
an entangled mesh and the nanofibers to fill the interstices
between said microfibers.
[0027] In effect, the microfiber interstices form the "pores" per
se, and the nanofibers fill in such openings to reduce the sizes
therein, and to a substantially uniform degree over the entire
nonwoven structure. Of highly unexpected benefit to the overall
invention, particularly in terms of targeting different levels of
porosity on demand, is the ability to dial in pore sizes within the
resultant nonwoven structure through the mere modification of the
concentration of microfibers to nanofibers alone. Thus, for
example, a 30% microfiber to 70% nanofiber proportion at the
nonwoven fabrication process outset would provide a pore size in
the range of 700 nm to 195 nm, whereas a 10% microfiber/90%
nanofiber combination would provide an effectively smaller pore
size distribution (as well as a more uniform range thereof, for
example 230 nm to 130 nm). Such an unforeseen result thus accords
an on-demand porosity result for the end user through, as noted, as
rather simple manufacturing modification. Such pore sizes created
can be measured, resulting in a mean flow pore size. Such mean flow
pore sizes may be less than 2000 nm, even less than 1000 nm,
preferably less than 700 nm, more preferably less than 500 nm.
[0028] Additionally, however, the manufacturer would subject the
nonwoven structure to a supercalendering operation in order to
effectively reduce the thickness thereof to extremely low levels,
as well as increase the density of the separator. A supercalender
is a stack of calender rolls, sometimes consisting of alternating
metal rolls and fiber-covered rolls, through which a sheet can be
passed through multiple nips. The rolls could also possibly be all
fiber-covered rolls, or all metal rolls, or any combination of
fiber-covered and metal rolls. The covering of fiber-covered rolls
is traditionally made from compressed paper or compressed cotton,
but another material of suitable compressibility may be used.
Traditionally, supercalendering is performed offline from the paper
machine, but the successive nips are in line with each other.
However, it may be possible to supercalender in-line with the paper
machine, or to do successive calendering operations to achieve
similar results.
[0029] The rolls generally are heated to increase the effect of the
pressure, though they can be used without adding heat. If heat is
used, it is preferential to use a temperature above 100 C, more
preferential above 125 C, and even more preferential above 150 C.
Pressure is applied to the nips through which the paper passes. A
suitable pressure is over 250 pounds per linear inch (pli),
preferentially above 500 pli, more preferentially above 1000 pli.
In addition, prior to supercalendering, the material is sometimes
moistened with water or other solvent to enable the fibers to
retain their calendered state better.
[0030] Prior work has been undertaken of straightforward
calendaring procedures for treatment of nonwoven separator
structures; such a process is limited in scope to application of
far lower pressures and forces on the subject nonwoven. In this
instance, the application of pressures and forces are significantly
higher than ever attempted on such nonwoven bi-component structures
in the past, particularly in terms of the utilization of at least
three calender nips at a minimum of 500 lbs/ft pressure. As a
result, typical calendaring approaches have resulted in thicknesses
of 250 microns, possibly as low as 50. In this situation, the
supercalendering operation suitably takes the already thin
structure and reduces the thickness (with concurrent density
increase) to at most about 25 microns, preferably lower (20, 15,
and even lower than 12 microns in measure). As noted above, the
capability of preventing contact between the anode and cathode of
the battery is necessary to prevent a shorted circuit during
battery use; the thickness of the separator and the controlled pore
size therein provide the essential manner of achieving such a
result. However, battery separator thickness may also contribute to
the available volume of other component parts within the closed
battery cell as well as the amount of electrolyte solution provided
therein. The entirety of the circumstances involved thus require an
effective separator in terms of multiple variables. The beneficial
ease of manufacture as well as the capability of providing
effective on-demand pore size and air resistance properties through
the inventive manufacturing method and the resultant single-layer
battery separator made from such a bi-component nonwoven structure
and subsequent supercalendering treatment thereto thus sets this
development distinctly apart from the state of the art battery
separators currently used and marketed today.
[0031] Additionally, it should be noted that although a
supercalendered single-layer separator including microfibers and
nanofibers together is encompassed within this invention, the
utilization of multiple layers of such a fabric structure, or of a
single layer of such an inventive battery separator fabric with at
least one other layer of a different type of fabric, may be
employed and still within the scope of the overall invention
described herein. Additionally, if desired, such separators may be
coated or otherwise treated with materials (such as ceramic sprays,
for example) that accord certain other properties to the structure
itself.
[0032] Such battery separators as described herein are clearly
useful for improving the art of primary and rechargeable batteries,
but also may be used for other forms of electrolyte conducting
energy storage techniques, such as capacitors, supercapacitors, and
ultracapacitors. Indeed, the control allowed on the pore size for
such inventive separators may allow significant improvements in the
energy loss, power discharge rate, and other properties of these
devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIGS. 1 and 2 are SEMs microphotographs of comparative prior
art nonwoven battery separator in a calendered state.
[0034] FIGS. 3 and 4 are SEMs microphotographs of inventive
supercalendered microfiber and nanofiber nonwoven fabric battery
separator.
[0035] FIG. 5 shows an exploded view of an inventive rechargeable
lithium ion battery including an inventive battery separator.
DETAILED DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS
[0036] All the features of this invention and its preferred
embodiments will be described in full detail in connection with the
following illustrative, but not limiting, drawings and examples. In
no manner has the description of the inventive separators and
battery cells made therewith been made herein in any attempt to
limit the scope thereof.
Microfiber and Nanofiber Production
[0037] As noted above, the microfiber may be constructed from any
polymer (or polymer blend) that accords suitable chemical and heat
resistance in conjunction with internal battery cell conditions, as
well as the capability to form suitable fiber structures within the
ranges indicated, and further the potential to be treated through a
fibrillation or like technique to increase the surface area of the
fibers themselves for entanglement facilitation during nonwoven
fabrication. Such fibers may be made from longstanding fiber
manufacturing methods such as melt spinning, wet spinning, solution
spinning, melt blowing and others. In addition, such fibers may
begin as bicomponent fibers and have their size and/or shape
reduced or changed through further processing, such as splittable
pie fibers, islands-in-the-sea fibers and others. Such fibers may
be cut to an appropriate length for further processing, such
lengths may be less than 1 inch, or less than 1/2 inch, or less
than 1/4 inch even. Such fibers may also be fibrillated into
smaller fibers or fibers that advantageously form wet laid nonwoven
fabrics.
[0038] Nanofibers for use in the current invention may be made
through several longstanding techniques to make nanofibers. One
example includes islands-in-the-sea, such as the NanoFront fiber
available from Teijin which are polyethylene terephthalate fibers
with a diameter of 700 nm. Hills also makes and sells equipment
that enables islands-in-the-sea nanofibers. Another example would
be centrifugal spinning. Dienes and FiberRio are both marketing
equipment which would provide nanofibers using the centrifugal
spinning technique. Another example is electrospinning, such as
practiced by DuPont, E-Spin Technologies, or on equipment marketed
for this purpose by Elmarco. Still another technique to make
nanofibers is to fibrillate them from film or from other fibers.
Nanofibers fibrillated from films are disclosed in U.S. Pat. Nos.
6,110,588, 6,432,347 and 6,432,532, which are incorporated herein
in their entirety by reference. Nanofibers fibrillated from other
fibers may be done so under high shear, abrasive treatment.
Nanofibers made from fibrillated cellulose and acrylic fibers are
marketed by Engineered Fiber Technologies under the brand name
EFTEC.TM.. Any such nanofibers may also be further processed
through cutting and high shear slurry processing to separate the
fibers an enable them for wet laid nonwoven processing. Such high
shear processing may or may not occur in the presence of the
required microfibers.
[0039] Nanofibers that are made from fibrillation in general have a
transverse aspect ratio that is different from one, such transverse
aspect ratio described in full in U.S. Pat. No. 6,110,588, which is
incorporated herein by reference. As such, in one preferred
embodiment, the nanofibers have a transverse aspect ratio of
>1.5:1, preferably >3.0:1, more preferably greater than
5.0:1.
[0040] As such, acrylic and polyolefin fibers are particularly
preferred for such a purpose, with fibrillated acrylic fibers, are
even more particularly preferred. Again, however, this is provided
solely as an indication of a potentially preferred type of polymer
for this purpose and is not intended to limit the scope of possible
polymeric materials or polymeric blends for such a purpose.
[0041] One particular embodiment of the combination of microfiber
and nanofibers is the EFTEC.TM. A-010-4 fibrillated
polyacrylonitrile fibers, which have high populations of nanofibers
as well as microfibers. Nonwoven sheets made of these materials are
shown in FIGS. 3 and 4. By way of example, these fibers can be used
as a base material, to which can be added further microfibers or
further nanofibers as a way of controlling the pore size and other
properties of the nonwoven fabric. Examples of such sheets with
additional microfibers added are shown in FIGS. 5, 6 and 7. Typical
properties of the acrylic Micro/Nanofibers are shown below.
TABLE-US-00001 TABLE 1 Acrylic Micro/Nanofiber Properties Density,
g/cm.sup.3 1.17 Tensile Strength, MPa 450 Modulus, GPa 6.0
Elongation, % 15 Typical Fiber Length, mm 4.5-6.5 Canadian Standard
Freeness, ml 10-700 BET Surface Area, m.sup.2/g 50 Moisture Regain,
% <2.0 Surface Charge Anionic
[0042] Such fibers are actually present in a pulp-like appearance
thereby facilitating introduction within a wetlaid nonwoven fabric
production scheme.
Nonwoven Separator Production Method
[0043] Material combinations were then measured out to provide
differing concentrations of both components prior to introduction
together into a wetlaid manufacturing process. Handsheets were made
according to TAPPI Test Method T-205, which is incorporated here by
reference. Several different combinations were produced to form
final nonwoven fabric structures.
[0044] Separators, both comparative and inventive, were then
prepared in accordance with the following examples:
[0045] Such separator examples were prepared using 0.5 denier
polyvinyl alcohol (PVA) fibers at 3 mm length and EFTec L-010-04
fibrillated lyocell nanofibers. The EFTec was dispersed using a
high speed industrial hydropulper, and then PVA fibers were mixed
in using a Valley Beater so that the final ratio of fiber materials
was 40% PVA and 60% Lyocell. This formed the pulp that was fed into
the paper machine.
[0046] The uncalendered paper was made using an industrial flat
wire paper machine such as is common in the industry, using common
papermaking settings for light weight sheets. This uncalendered
material is Comparative Example 1 and the properties of such a
material are in Table 1, below.
[0047] Part of the separator material was then calendered at 300 C
and 300 m/min between a steel roll and a hard rubber roll at
pressure 1000 lbs/inch. This is Comparative Example 2, and the
properties are shown in Table 1, below, as well.
[0048] Another part of the material was then also furthered
calendered on a supercalender at 300 C, at 100 m/min, at stack
pressure of 1500 lbs/inch. The supercalender consists of 4 nips,
each between steel and hard rubber rolls. This supercalendered
material is Inventive Example 1, and the properties are shown in
Table 1, below.
TABLE-US-00002 TABLE 1 Measurements of Separators Comparative
Comparative Inventive Units Example 1 Example 2 Example 1 PVA % 40%
40% 40% L-010-04 % 60% 60% 60% Average Material Density g/cm.sup.3
1.36 1.36 1.36 Basis Weight g/cm.sup.2 19.78 19.78 19.75 Moisture %
6.9 6.1 7.3 Thickness (7.3 psi) .mu.m 48 34 21 Thickness (12.6 psi)
.mu.m 46 32 19 Thickness (25 psi) .mu.m 43 32 19 Apparent Density
(7.3 psi) g/cm.sup.3 0.412 0.582 0.940 Apparent Density (12.6 psi)
g/cm.sup.3 0.43 0.618 1.039 Apparent Density (25 psi) g/cm.sup.3
0.46 0.618 1.039 Porosity (7.3 psi) % 70% 57% 31% Porosity (12.6
psi) % 68% 55% 24% Porosity (25 psi) % 66% 55% 24% MD Tensile
Strength kN/m 0.48 0.58 0.86 MD Tensile Strength (12.6 psi)
kN/cm.sup.2 1.04 1.81 4.53 MD Tensile Stretch % 1.89 2.63 2.33 CD
Tensile Strength kN/m 0.31 0.33 CD Tensile Strength (12.6 psi)
kN/cm.sup.2 0.67 1.03 CD Tensile Stretch % 2.80 3.13 Gurley Sec/100
cc 35 63 261 Mean Flow Pore Size .mu.m 0.85 0.76 0.52 Bubble Point
.mu.m 3.55 6.28 5.08
These measurements are defined as follows:
[0049] a) Average material density is 1/(% PVA/dens(PVA)+%
L/dens(L)), where % PVA is the proportion of PVA fiber, the
dens(PVA) is the density of PVA in g/cm.sup.3, the % L is the
proportion of lyocell (in %) and the dens(L) is the density of
lyocell in g/cm.sup.3.
[0050] b) Apparent density is Basis weight/thickness as measured at
a given foot pressure
[0051] c) Porosity is 1--Apparent density/Average material
density
[0052] d) Tensile Strength (12.6 psi) is Tensile Strength/Thickness
where the thickness used is that measured at a foot pressure of
12.6 psi.
[0053] From the above table it is evident that the inventive
separator is stronger, is thinner, has lower pore size, and has
lower porosity and higher density than the comparative examples.
One key benefit is the reduced amount of electrolyte necessary to
fill the separator due to the greatly reduced porosity and
thickness, greatly reducing the cost of the materials that go into
the supercapacitor.
[0054] In terms of the inventive separators made herein through the
supercalendering process, the following Table 2 provides parameters
(ranges) of the thickness, porosity, mean flow pore size, tensile
strengths, apparent density, fiber size, fiber length, calender
nips, and calender pressures pertaining to manufacture thereof.
TABLE-US-00003 TABLE 2 Property Ranges of Inventive Separators More
Most Parameter Units Qualifier Range Preferably Preferably
Preferably Thickness .mu.m Below 25 20 15 12 Porosity % Below 45 40
35 30 Mean Flow Pore Size .mu.m Below 0.7 0.6 0.5 0.4 Tensile
Strength kN/cm.sup.2 Above 2 2.5 3 3.5 Tensile Strength kN/m Above
0.6 0.7 0.8 0.85 Apparent Density g/cm.sup.3 Above 0.7 0.8 0.9 1.0
Fiber size .mu.m Below 2 1 0.7 0.5 Fiber length mm Below 25 12 8 5
Calender nips # Above 3 4 5 Calender pressure Lbs/inch Above 500
1000 1500 2000
[0055] Thus, from this table, the thickness should be below 25
microns, preferably below 20 microns, more preferably below 15
microns, and most preferably below 12 microns. Each of the other
parameters can be read in a parallel fashion.
[0056] The comparative and inventive separators were further
analyzed under scanning electron microscopy. SEM micrographs were
taken of the separators for Comparative Example 2, and Inventive
Example 1, and are shown below. It is clear that the inventive
separator shows significant differences in physical appearance,
particularly in terms of higher density, lower porosity and smaller
pore size. These micrographs are shown in FIGS. 1 and 2.
Battery Separator Base Analysis; Super Capacitor and Lithium Ion
Battery Testing
[0057] Supercapacitors were prepared using a commercial separator,
TF 4030 available from Nippon Kodashi (NKK), Comparative Example 2,
above, and Inventive Example 1, above. The supercapacitor
electrodes were obtained by disassembling a Maxwell 3000 F
supercapacitor and washing the electrodes with acetonitrile, then
drying. Cells were made by cutting electrodes into approximately
2''.times.3'' plates, and then making a sandwich of two electrodes
with the separator in the middle. The sandwich was placed in a
pouch, filled with electrolyte consisting of 1 M
(tetraethylammonium tetrafluoroborate) salt in acetonitrile
solvent, and sealed. The cells were then charged to 2.8 V and held
for 24 hours before testing for capacity at 7 mA and at 70 mA
(approximately 12 mAh cells). The ESR was obtained by measuring the
instant voltage drop at the specified current. Each cell type was
prepared in duplicate, and the average of the two cells is shown
below in Table 2.
TABLE-US-00004 TABLE 3 Supercapacitor Property Measurements
Comparative Comparative Inventive Supercapacitor Supercapacitor
Supercapacitor Units Example 1 Example 2 Example 2 Electrodes
Maxwell Maxwell Maxwell Maxwell Separator NKK TF4030 Comparative
Example 1 Example 2 Capacity (7 mA) mAh 11.97 12.56 12.262 Capacity
(70 mA) mAh 10.35 10.09 9.60 ESR Ohms 0.167 0.170 0.221 Volume
Capacity (7 mA) mAh/cm.sup.3 11.66 12.22 12.53 Volume Capacity (70
mA) mAh/cm.sup.3 10.07 9.83 9.80
[0058] For these measurements, Volume Capacity equals
Capacity/(0.244 mm+sep thickness(mm))*75 mm*25 mm, where "sep
thickness(mm)" is the separator thickness in mm. These measurements
show the increase in ESR with comparable capacity and volume
capacity levels, showing effectiveness, at least, with an improved
performance in this manner for the inventive separator
supercapacitor.
[0059] Separators for lithium batteries were also prepared by
blending EFTec L-010-04, EFTec A-010-04 and 0.3 denier polyethylene
terephthalate fibers cut to 5 mm at a 40:40:20 blend ratio, with
the separators prepared similarly to Comparative Example 2, above,
this being Comparative Example 3. Additionally, the same
uncalendered material was supercalendered according to the same
procedure as Inventive Example 1, this on being Inventive Example
2. The material properties of these materials are shown below in
Table 4.
TABLE-US-00005 TABLE 4 Measurements of Separators Comparative
Inventive Test Units Example 3 Example 2 PET (0.3 dpf, 5 mm) % 20
20 L-010-04 % 40 40 A-010-04 % 40 40 Average Material Density
g/cm.sup.3 1.34 1.34 Basis Wt g/m.sup.2 17.05 17.73 Moisture % 2.45
3.74 Dry Basis Wt g/m.sup.2 16.6 17.1 Thickness 7 psi .mu.m 28 21
Thickness 12 psi .mu.m 27 19 Thickness 25 psi .mu.m 26 18 Apparent
Density (7 psi) g/cm.sup.3 0.609 0.844 Apparent Density (12 psi)
g/cm.sup.3 0.631 0.933 Apparent Density (25 psi) g/cm.sup.3 0.656
0.952 Porosity (7 psi) % 55% 37% Porosity (12 psi) % 53% 30%
Porosity (25 psi) % 51% 29% MD Tensile Strength kN/m 0.74 0.55 MD
Tensile Strength (12.6 psi) kN/cm.sup.2 2.74 2.89 MD Tensile
Stretch % CD Tensile Strength kN/m 0.44 0.38 CD Tensile Strength
(12.6 psi) kN/cm.sup.2 1.63 2.00 CD Tensile Stretch % Breaking
length % 3.0 1.6 Young's Modulus Ksi 333 374 Gurley Sec 82 374 Mean
Flow Pore Size .mu.m 1.11 0.539 Bubble Point .mu.m 22.45 3.164
[0060] Lithium batteries were also prepared by making a stack of
ten double sided electrodes, the cathode consisting of lithium iron
phosphate on aluminum and the anode consisting of graphite on
copper, each cathode and anode pair separated by a layer of
separator. The stacks were placed in a pouch and saturated with
electrolyte, then sealed. The electrolyte used was a mixture of
ethylene carbonate, dimethyl carbonate and diethyl carbonate mixed
in a 4:3:3 volume ratio, with 1 mol/1 of LiPF.sub.6 salt. The
batteries had a design capacity of 3 Ah, and were charged at C/6 to
3.6 V at constant current, then charged at constant voltage until
the current reached 50 mAh. They were then discharged at a constant
current of 500 mAh. Two more identical charge and discharge cycles
were performed, with the third charge taken as the capacity of the
cell. The cell was then charged to 900 mAh at constant current of
500 mA, and left to rest for 24 hours, with the voltage recorded at
the beginning and end of the rest. The difference in these voltages
was taken to be the 24 hour self-discharge. Cells were prepared
with Celgard 2500 separator (Comparative Example 5), Dreamweaver
Silver 25 separator (Comparative Example 6), Inventive Example 1
(Inventive Battery Example 1) and Inventive Example 2 (Inventive
Battery Example 2). The results are shown in Table 3, below. In
each case, two cells were prepared and the results averaged. FIG. 3
shows an example of the different components of a lithium ion
battery, as well, that may be an embodiment of this invention.
TABLE-US-00006 TABLE 5 Lithium Battery Property Measurements
Comparative Comparative Inventive Battery Inventive Battery Units
Example 5 Example 6 Example 1 Example 2 Separator Celgard
Dreamweaver Inventive Inventive 2500 Silver 25 Example 1 Example 2
Cell Capacity mAh 2852 2974 2936 2897 Self Discharge Loss mV 37.2
30.6 71.9 39.9
[0061] The results thus show the improvements accorded through the
thin, high density nonwoven bi-component battery separators in
comparison with typical structures now utilized within the
industry.
[0062] It should be understood that various modifications within
the scope of this invention can be made by one of ordinary skill in
the art without departing from the spirit thereof. It is therefore
wished that this invention be defined by the scope of the appended
claims as broadly as the prior art will permit, and in view of the
specification if need be.
* * * * *